Handheld power tools tend to be small, lightweight and portable, making them useful in many situations. However, these tools can also be dangerous, in that they may continue to operate even after the user has turned it off. For example, momentum may cause the grinding wheel on a reversible grinder to rotate even after the user has released the trigger. To alleviate the problem, many power tools incorporate a dynamic brake and provide a default position for the power switch that engages the dynamic brake. Dynamic brakes typically short the motor windings through a resistor. In order to increase the amount of braking power, it is known to sometimes employ an auxiliary set of windings in series with the motor windings.
U.S. Pat. No. 5,600,107 to Tsai and U.S. Pat. No. 5,380,971 to Bittel disclose trigger mechanisms for power tools with dynamic braking systems. In both patents, depressing or squeezing the trigger moves the electrical contacts within the switch, completing a power circuit and operating the tool's motor. The triggers are spring-biased to the OFF position. Releasing the trigger causes the electrical contacts to move back to the OFF position, both breaking the motor supply circuit and completing a separate dynamic braking circuit. Thus, two sets of contacts are required to move before the dynamic brake is activated and the motor stops. Neither patent is applicable to reversible power tools.
A typical reversible power tool directional switch has FORWARD, OFF and REVERSE positions. In the OFF position, current flow through the motor windings is interrupted. However, in order for a dynamic brake to operate, a current path through the motor windings must be maintained.
U.S. Pat. No. 5,892,885 to Smith discloses a complicated dynamic braking circuit, for use with a reversible power tool. The dynamic brake is engaged by disconnecting the power source and speed control circuit, then connecting two other contacts, creating a low resistance path between the motor terminals. The means for switching the direction of operation of the motor comprises a pivoting lever that is independent of the actuating trigger. It therefore appears that it is possible, using the switch disclosed by Smith, to switch the direction of tool operation between FORWARD and REVERSE without any intermediate state, thereby presenting a significant risk to the user.
U.S. Pat. No. 5,638,945 to Fukinuki discloses a safety mechanism in the form of a lock or blocking element associated with the trigger, so as to physically prevent the trigger from moving out of position until the user does so intentionally by releasing the lock. A sliding safety switch is disposed on opposite sides of a tool handle. The safety switch consists of a generally T-shaped sliding lock button that is spring-biased to return to the locked position upon release by the user. An abutment surface extends from the trigger and physically blocks the sliding switch, preventing accidental depression of the trigger and operation of the tool. Once the sliding switch is moved to an operational position, it no longer blocks the trigger abutment surface. When the trigger is depressed, a hooked extension at the front of the trigger engages an inversely hooked extension at the front of the sliding switch. This contact locks the tool in an operative mode by preventing the switch from sliding back to a neutral position, even if the trigger is released. Depressing the trigger again unlocks the hook, allowing the tool to shut down when the trigger is released. While the hook mechanism reduces hand fatigue by allowing the user to operate the tool without maintaining pressure on the handle, if the user accidentally drops the tool while the hook is engaged, the tool will continue to operate. This poses a significant danger to both the user and to any person or thing around him. Further, the switch mechanism disclosed by Fukinuki is unsuitable for use in a power tool with a dynamic brake, as the switch does not actively make or break an electrical contact, as is required for a dynamic braking system.
It is an object of the present invention to provide a switch mechanism that is suited for a reversible power tool having a dynamic brake. The switch arrangement according to the invention is designed to maintain a current path through the motor windings when a FORWARD, OFF and REVERSE switch is moved to the OFF position. There is also provided a safety mechanism to ensure that the dynamic brake is engaged prior to switching between the FORWARD and REVERSE positions.
The foregoing and other objects of the invention will be appreciated by reference to the summary of the invention and to the detailed description of the preferred embodiment that follow.